Directional elements have been developed that process transverse electromagnetic waves. For example, directional radio frequency antennas (e.g., a parabolic dish antenna) and directional optical elements (e.g., lasers and CCDs) have been developed. Directional elements have also been developed that process longitudinal waves. For example, directional microphones have been developed. Characteristic of directional elements is a boresight, the axis of maximum gain with respect to the signal being processed by the element. The boresight of a directional antenna is the axis of maximum gain in the antenna's radiation pattern. For example, in an axially-fed parabolic dish antenna, the boresight is the axis of symmetry of the parabolic dish. Many applications for directional elements require that the boresight of the element be adjustable. For example, if a directional antenna is used to track a moving object, the position of the boresight of the directional antenna typically must be moved to keep the moving object within the radiation pattern at or near the boresight.
To move a directional element, a mast is employed that is capable of moving the boresight of the element within some defined range. Typically, such masts employ a gimbal mechanism to facilitate the positioning of the boresight of the element. The gimbal mechanism extends from a first end that is attached to a base to a second end that is attached to the directional element structure. Associated with the gimbal mechanism is an x-y-z orthogonal coordinate system. Rotation about the x, y, and z axes can respectively be defined as pitch, yaw, and roll. The gimbal mechanism typically includes two gimbals, the first gimbal providing the ability to roll the directional element structure within a defined range and the second gimbal providing the ability to pitch/yaw the directional element structure within a define range. The range of motion of the first and second gimbals defines the spherical section within which the boresight of the directional element can be positioned. Typically, the first gimbal supports the second gimbal and the second gimbal supports the directional element. Further, the first gimbal also typically supports the motor used to rotate the directional element about the second gimbal. Consequently, the motor used to rotate the first gimbal must rotate the first gimbal, second gimbal, directional element, and motor for rotating the directional element about the second gimbal.
The volume needed to accommodate a directional element and a gimbal mechanism for positioning the boresight of the directional element is directly proportional to the dimensions of the directional element and the extent of the spherical section within which the boresight can be positioned. For example, as the dimensions of a directional antenna increase with the spherical extent being held constant, the greater the volume needed to accommodate the antenna and gimbal mechanism. Likewise, as the spherical extent increases with the dimensions of antenna being held constant, the greater the volume needed to accommodate the antenna structure and gimbal mechanism. Of particular concern in many applications is the height of this volume. For example, when a directional antenna and gimbal are disposed substantially outside the typical exterior surface of an aircraft (typically, under some kind of cover), the height of the volume occupied by the antenna and gimbal mechanism typically increases drag and/or changes the performance of the aircraft. Further, the height of the antenna and gimbal mechanism (or related cover) also creates a visual signature that is undesirable in particular instances.